This invention relates to an overload warning device for use in mine roof construction and more specifically to a warning system capable of providing an audible warning signal indicating structural deficiency in the roof of a mine.
In underground mining there is constant danger both to the mine workers and the expensive mining machinery of a shift in the strata above the mine passage or tunnel eventually leading to a cave in if precautionary measures are not taken.
Minor shifts of the strata in the mine roof indicative of pending disaster are not always noticeable to the naked eye of even an experienced mining engineer. The use of bolts, instead of supporting timbers, to hold the mine roofs in place are well known in the art. The bolts are inserted into holes that have been drilled into the roof and the upper ends of the bolts are expanded in some manner to anchor them in place. The bolts are further provided with a mounting plate which abuts against the mine roof. The bolts are tightened to bolt the various strata of the roof together and to cause expansion of the anchor means against the interior of the drilled hole. Tension is imparted to the bolt by drawing up the bolt head in a tight abutment with the lower surface mounting plate. Nevertheless, in time the strata may start to separate and develop a load on the bolt. If the load becomes great enough, the bolts will stretch and fail without warning, thereby allowing the roof to collapse.
It is highly desirable to have an early warning system established for detecting the dangerous conditions which can develop in the roof of a mine. Various mine roof warning devices have previously been suggested which respond to the shifts in the strata of the roof of a mine such that some form of signal means is activated to alert pending disaster. For example, elaborate sensing devices which provide visual signs of increased pressures being applied to the ceiling bolts such as in the form of flag warning systems, the installation of a series of weakened bolts, the ruptured bolts being visually detected on the floor of the mine, and various battery operated or electrically actuated signal devices, which are either inefficient, impractical or expensive, have been disclosed.
While, as stated above, the practice of bolting roof plates to the rock structure of the roof of the tunnel is widely used to inhibit the shifting of the rock, occasionally the rock formation will shift regardless of the presence of the bolts, causing a very dangerous situation. In this regard, such a shift in the rock formation may result in the collapse of the mine tunnel roof entirely. It is therefore, imperative to have a positive and reliable means for ascertaining any shift in the rock formation above the tunnel roof.
When the rock formation above the tunnel roof shifts, a resulting stress is placed upon the bolts which are located within the rock, the tensile stress imparted thereto being transferred to a bolt head generally secured to the end of the bolts which is visible from within the tunnel in close proximity to a metal roof plate. A measurement of the increase of the stress imparted to the mine roof supporting bolt translated into the form of a broken weaker bolt, a flag or an electrically generated signal as a means for indicating dangerous shifts in the rock formation can be recorded. Such means for indicating the shifting of rock formations will only be as reliable as the means for indicating the added stresses imparted to the bolt.
When the detection device relies solely upon visual means, the weakening of the roof of the mine and the impending rupture of the bolt may be overlooked due to the environment in which the workers are employed, intensity of the concentration of the worker or just carelessness. In order to overcome these disadvantages, a truly efficient and practical indicating means would provide an audible signal which clearly identifies danger stresses imparted to the bolts without requiring critical measurements as to stresses being imparted from within the strata or conscious observation on the part of the working personnel.